On the abundance gradients of organic molecules along the TMC-1 ridge

Gradients in molecular abundances along the TMC-1 ridge have been observed by several authors, most recently in a comprehensive study by Pratap et al. These can be explained by there being a difference in density, C/O ratio, or chemical evolutionary state along the ridge. The presence at the carbon-rich cyanopolyyne peak (CP) of specific oxygen-bearing molecules, believed to be products of grain surface reactions, leads us to investigate the possibility that the gradients are produced following the removal of icy grain mantles. We identify the mantle removal mechanism as the explosive desorption of photolyzed ices, induced by MHD waves as they propagate within the cloud. By identifying the protostellar object IRAS 03381+2540 as the source of these waves, we have constructed a dynamical-chemical model for the evolution of the TMC-1. ridge. The results of detailed chemical kinetic calculations are presented. We find that injection into the gas phase of the carbon-bearing species C2H2, C2H4 and CH4 can transiently enhance the production of many organic molecules, particularly the cyanopolyynes and polyacetylenes, and hence that the observed gradients can indeed be produced in this model. We suggest that other mantle-driven reaction pathways, involving formation of methylated chains (e.g., CH3C3N) by gas phase methyl cation transfer, as well as formation of carbenes and suboxides by fragmentation of surface-formed cumulenone compounds, might explain the presence of many other organic molecules in TMC-1. In our view the current chemical state of TMC-1 reflects a transient phase that accompanies the earliest epochs of low-mass formation and that regions rich in organic molecules should be observable around known Class 0 protostellar sources and be offset from them by about an ion-neutral damping length. Molecular clumps in dense cores should also show structure on this scale-length.